Loud noise exposure experienced during military service results in the two most common service-related disabilities faced by Veterans ? tinnitus and hearing loss. Recent animal studies indicate noise exposure can result in significant loss of auditory nerve synapses. Additional studies suggest that synaptic loss may be associated with tinnitus and hyperacusis. However, synaptic loss can exist even when auditory thresholds are normal, making it difficult to detect. Since clinical audiometric testing is not sensitive to this ?hidden hearing loss?, we currently have no method of testing people for this type of auditory damage. This indicates that there is a critical need to identify individuals suffering from noise-induced cochlear synaptic degeneration and the related perceptual deficits. The long-term goal is to develop strategies for prevention, monitoring and treatment of hidden hearing loss in the Veteran population. The overall objective for this CDA-2 application is to identify physiological markers of hidden hearing loss. The central hypothesis is that individuals with a significant history of noise exposure will show physiological differences in the auditory nerve, brainstem, and cortex when compared to individuals with less noise exposure and that these differences will be associated with hyperacusis and tinnitus. This hypothesis is based on preliminary data and the research literature. Preliminary data shows differences in the amplitude of wave I of the auditory brainstem response (a measure of the synchronous firing of the auditory nerve) between individuals with differing levels of noise exposure. The research literature suggests that noise exposure and tinnitus/hyperacusis may be associated with additional changes in auditory physiology. The rationale that underlies this proposal is that identification of physiological markers of hidden hearing loss and the resulting perceptual changes will facilitate the development of methods for preventing or treating this condition. This hypothesis will be tested by pursuing the following four specific aims: 1) Identify envelope following response (a measure of the brainstem's ability to phase-lock to the envelope of a stimulus) abnormalities present in Veterans with normal pure tone thresholds and high levels of noise exposure; 2) Identify elements of the middle and late latency responses (auditory evoked potentials generated by the cortex) that differentiate noise-exposed Veterans with normal auditory thresholds from individuals with less noise exposure; 3) Evaluate Veterans with high noise exposure and normal pure tone thresholds for differences in contralateral suppression of distortion product otoacoustic emissions (a measure of the strength of the medial olivocochlear efferent feedback pathway); and 4) Investigate the relationship between hyperacusis and tinnitus and the physiological measures described in aims 1-3. The physiological measures described above will be compared across three groups: Veterans with a history of exposure to high levels of noise, Veterans with less exposure, and non-Veterans with limited noise exposure. To minimize the effects of age and hair cell loss, only young subjects with normal pure tone thresholds will be included. This approach is innovative because it investigates noise-related changes to auditory physiological measures in Veterans, a unique population of individuals with significant noise exposure history. This contribution will be significant because it will identify metrics to detect hidden hearing loss, providing the basis for a clinical test battery that can be used to aid in the prevention and treatment of cochlear synaptic degeneration in the Veteran population.